The field of microrobotics seeks to construct mobile robots able to sense, move through, and manipulate their environment with dimensions on the order of millimeters or smaller. Microrobots have been constructed that roll, walk, swim, and fly. Microrobots are particularly useful because they can go to places other robots cannot, such as into the rubble at a disaster site, into a machine to repair it, or into the body for minimally invasive surgery. A major problem in microrobot design is supplying enough power for long run time. In the fields of microrobotics and programmable matter, there is therefore a need for actuators capable of electromechanical energy conversion at high torque and low speed, and capable of scaling to small dimensions without loss of efficiency.
The emerging field of programmable matter seeks to build macroscopic objects with thousands of actuatable degrees of freedom, so that their shape and function may be changed under software control. Like microrobots, programmable matter will also require efficient small-scale actuators.
A wide variety of micromotors and microactuators have been constructed that have their largest dimension measured in millimeters. These actuators have used a variety of operational principles, including magnetic, electrostatic, piezoelectric, and electrothermal. Because it is difficult and expensive to fabricate efficient speed-reducing power transmissions at microscale, an actuator that directly produces the high torques at low speeds efficiently would be desirable for robotics and programmable matter. Such an actuator could be advantageously be applied even with a speed-reducing (or speed-increasing) power transmission, but the transmission might not require as many stages, have as large a speed ratio, or as large a maximum speed as it would otherwise.
Magnetic motors dominate electrical to mechanical energy conversion at macroscale, powering a wide range of devices from industrial machine tools to household appliances to children's toys. These motors are available in a wide range of types (e.g. induction, servo, stepper) and configurations (e.g. rotary, linear) depending on the requirements of the application. Magnetic motors have two halves, a rotor and a stator. Electrical energy provided from an external source is used to produce a changing magnetic field at the interface between the rotor and stator, which propels the rotor and produces useful mechanical work.
Since the construction of the first electric motor by Michael Faraday in 1821, permanent magnets have been used in the construction of motors. In small electric motors (having a capacity less than one horsepower) permanent magnets are commonly used in order to improve efficiency and torque, and to simplify construction and drive. For example, in the brush-commutated permanent-magnet DC motor, a periodically-reversing current through coils on the rotor interacts with the magnetic field of permanent magnets on the stator to produce torque. A key design criterion for almost all permanent magnet motors is that the demagnetizing field inside the permanent magnets must not exceed a the demagnetizing threshold. Otherwise, the magnetic field of the magnets will be reduced by operation, and the motor will not function as well. Conventional permanent magnet motors are carefully designed to avoid demagnetization of their permanent magnets.
A notable exception is the hysteresis motor (such as, for example, that disclosed in U.S. Pat. No. 3,610,978) which works by continuously cycling a piece of magnetic material around its hysteresis loop, and generating continuous torque due to the time lag between field and flux while changing the magnetization of the material. The hysteresis motor is notable and useful for its ability to produce constant torque independent of speed. However, the hysteresis motor requires the continuous input of electric power to produce continuous torque, even at zero speed, and so has low efficiency at low speeds.
Electric motors suffer from a number of loss mechanisms that act to reduce their power efficiency. At high speeds, mechanical losses due to friction, and magnetic losses due to cyclic magnetization and demagnetization of the flux-carrying members dominate. Thus, flux-carrying materials (e.g. iron) for magnetic motors are typically selected to have the lowest possible coercivity and thus the lowest possible magnetic losses. At low speeds, loss due to resistive heating of the coils dominates. At the limit of zero speed, when the motor is stalled, 100% of the electrical energy input goes to resistive heating of the windings and the motor operates at zero percent efficiency.